Linear accelerators (LINACs) are useful tools for industrial applications, such as radiography, cargo inspection and food sterilization, and medical applications, such as radiation therapy and imaging. In some of these applications, beams of electrons accelerated by the LINAC are directed at the sample or object of interest for analysis or for performing a procedure. However, in many of these applications, it can be preferable to use x-rays to perform the analysis or procedure. These x-rays may be generated by directing the electron beams from the LINAC at an x-ray emitting target.
A cargo inspection device that uses x-rays generated from a LINAC is useful during non-intrusive inspection of cargo because of the high energy output (and therefore greater penetration) that it provides. As a result, large quantities of containers may inspected more accurately without requiring inspectors to open the containers.
Typically, the LINACs used in cargo inspection systems are configured to produce a single energy x-ray beam. A detector receives the single energy x-ray beam that has penetrated the shipping container without being absorbed or scattered, and produces an image of the contents of the shipping container. The image may be displayed to an inspector who can perform visual inspection of the contents. The inspector may observe contents in the container that require further analysis. It has been suggested to vary the x-ray dosage, i.e., intensity, to further inspect dense cargo. It would be desirable to provide a LINAC based x-ray source configured to modulate pulse-to-pulse intensity while outputting energy stable electron beams from the LINAC.
Other previously-known cargo inspection devices use dual energy LINACs that are configured to emit two different energy level x-ray beams. With a dual energy x-ray inspection system, materials can be discriminated radiographically by alternately irradiating an object with x-ray beams of two different energies. Dual energy x-ray inspection systems can determine a material's mass absorption coefficient, and therefore the effective atomic (Z) number of the material. Differentiation is achieved by comparing the attenuation ratio obtained from irradiating the container with low-energy x-rays to the attenuation ratio obtained from irradiating the container with high-energy x-rays. Discrimination is possible because different materials have different degrees of attenuation for high-energy x-rays and low-energy x-rays, and that allows identification of low-Z-number materials (such as but not limited to organic materials), medium-Z-number materials (such as but not limited to transition metals), and high-Z-number materials (such as but not limited to radioactive materials) in the container. Such systems can therefore provide an image of the cargo contents and identify the materials that comprise the cargo contents.
The ability of dual energy x-ray inspection systems to detect the Z number of materials being scanned enables such inspection systems to automatically detect the different materials in a container, including radioactive materials and contraband such as but not limited to cocaine and marijuana. However, conventional dual energy x-ray inspection systems use a standing wave LINAC that is vulnerable to frequency and power jitter and temperature fluctuations, causing the beam energy from the linear accelerator to be unstable when operated to accelerate electrons to a low energy. The energy jitter and fluctuations can create image artifacts, which cause an improper Z number of a scanned material to be identified. This can cause false positives (in which a targeted material is identified even though no targeted material is present) and false negatives (in which a targeted material is not identified even though targeted material is present).
Like single energy x-ray inspection systems, dual energy x-ray inspection systems may produce an image of the contents of a shipping container that may be displayed to an inspector who can perform visual inspection of the contents. The inspector may observe contents in the container that require further analysis. Accordingly, it would be desirable to provide a dual energy LINAC based x-ray inspection system configured to modulate pulse-to-pulse intensity to increase an inspector's ability to accurately investigate cargo.